The electronics industry is continuously striving to create electronic devices that are smaller, cheaper, and provide higher resolution performance. As such, it has become necessary to develop new photoimageable compositions and processes for photopatterning which accomplish these goals.
Photo-patterning technologies offer uniform finer lines and space resolution when compared to traditional screen-printing methods. A photo-patterning method, such as DuPont's FODEL® photoimageable thick film pastes, utilizes a photo-imagable organic medium as found in U.S. Pat. No. 4,912,019; U.S. Pat. No. 4,925,771, U.S. Pat. No. 5,049,480, U.S. Pat. No. 5,851,732 and U.S. Pat. No. 6,075,319 whereby the substrate is first completely covered (printed, sprayed, coated or laminated) with the photo-imagable thick film composition and dried, if necessary. An image of the pattern is generated by exposure of the photo-imagable thick film composition with actinic radiation through a photomask bearing a pattern. The exposed substrate is then developed. The unexposed portion of the pattern is washed away leaving the photo-imaged thick film composition on the substrate that is then fired to remove organic materials and sinter inorganic materials. Such a photo-patterning method demonstrates thick film line resolution of about 30 microns or larger depending on the substrate smoothness, inorganic particle size distribution, exposure and development variables. It has been proven that such a technique is useful in the manufacture of flat panel displays, such as plasma display panels.
However, the above-mentioned photo-imagable thick film compositions and tape compositions are all based on free-radical polymerization, where acrylate or methacrylate monomers are polymerized by initiation of free radicals that are in turn generated by decomposition of photoinitiators. The polymerized acrylate or methacrylate monomers make UV-exposed area less soluble.
Free radical polymerization can be inhibited by oxygen in air or in thick film compositions. Currently, almost all PDP makers use a so-called off-contact way to expose photo-imagable thick film composition to avoid possible damage of photomasks. Quite often, thick film compositions are tacky to some extent. If a photo-mask contacts the composition, part of the composition could get onto the photo-mask and damage the images on the mask. The ‘off-contact’ means that there is gap between photo-mask with desired electronic circuitry pattern and thick film composition to be exposed. The gap is filled with air, i.e. with the presence of oxygen. This is one of the causes for processing variability.
The present invention relates to a composition(s) with cationically polymerizable monomers (including epoxy monomers), method of photo-patterning, and method of forming an electrode. For the compositions presented by this invention, UV-exposure is no longer sensitive to the presence of oxygen between the gap of the off-contact exposure. This invention discloses novel thick film paste and tape compositions, methods of photo-patterning, and electrode formation. The thick film paste compositions contain epoxide monomers and onium salt photoinitiators. Onium salts decompose upon actinic UV-radiation to generate super acids or cations, which in turn initiate the polymerization of epoxide monomers. The polymerized epoxide monomers make UV-exposed areas less soluble and remain as electronic circuitry pattern on a substrate after development.